A conventional VTR records and reproduces video and audio signals using respective dedicated tracks. On the other hand, a hi-fi VTR records and reproduces video and audio signals by superimposing these signals on one track. As compared with a conventional video signal, the superposition recording mode employed in the hi-fi VTR does an in-depth recording of the audio signal. More specifically, the audio signal is first recorded on the track and then the video signal is superimposed thereon. This recording sequence is due to a difference in frequency between the video and audio signals. The frequency of the video signal is about 3.4.about.4 MHz, which is higher than that of the audio signal, which is either 1.3 MHz or 1.7 MHz. The superimposing of signals during the recording mode substantially differentiates a hi-fi VTR from a conventional VTR.
The hi-fi VTR experiences problems due to the recording mode characteristics described above because the video and audio signals have a relative effect on each other caused by their respective recording currents. In other words, when a video signal is superimposed on the track on which an audio signal is recorded, once the recording current increase of the video signal goes beyond a certain level, it has an effect on the magnetizing state of the audio signal on the tape so that the signal-to-noise ratio (SNR) of the reproducing output is degraded and noise occurs when listening. In contrast, once the recording current increase of the audio signal goes beyond a certain level, the difference in frequency between the L-CH (1.3 MHz) and the R-CH (1.7 MHz), i.e., 400 kHz, has an effect on the color signal of the video signal, e.g., 629 kHz. Therefore, a beat occurs in the video. The mutual interference between video and audio signals as described above is well known.
FIG. 1A illustrates level variation for audio signals with 1.7 MHz (L-CH) and 1.3 MHz (R-CH) bandwidths for the exemplary case where the write current of the video signal gradually increases. In other words, when the reproducing output of the audio signals has been set so that it can be obtained at the maximum level and the audio and video signals are superimposed as described above, the curves in FIG. 1A illustrate audio and video signals which represent respective reproducing levels at their recording currents. Here, the reproducing output is an FM envelope of a reproduced signal. The curve (a) in FIG. 1A represents a video signal having a bandwidth of approximately 4 MHz as previously described, while curves (b) and (c) represent the above-mentioned audio signals, i.e., curve (b) represents the left channel (L-CH) having a bandwidth of approximately 1.3 MHz while curve (c) represents the right channel (R-CH) having a bandwidth of approximately 1.7 MHz.
As shown in FIG. 1A, when the recording current of a video signal goes higher than point A0, the reproducing level of the video signal increases to some extent, while the reproducing level of the audio signals decreases. In this manner, when the reproducing level of an audio signal decreases, the SNR of the audio signal is degraded and noise occurs, as described above.
Conventionally, to improve the above-mentioned drawback, that is, the mutual interference due to the magnitude of the write current between an audio and a video signal, the write current of an audio signal has been limited. In other words, the A0 point, as shown in FIG. 1A, has been established as an optimum point, at which mutual interference between the audio and video signals does not occur, and then the write current applied to an audio head is adjusted so as to be applied at point A0 or lower.
This limitation on A0 prevents the occurrence of a beat in the video signal, but causes the dynamic range for the audio signal to be reduced, thereby degrading the audio reproduction level. More particularly, the maximum recording current of the audio signal in an exclusive audio write control operation is applied with respect to the set point, the point A0 mentioned above, resulting in the optimum reproducing level for listening being unobtainable. FIG. 1B, which shows the ratio between a reproducing level and a write control operation, illustrates the level of the reproducing output of the audio signal increases depending on the increase of its write current, but its reproducing output is limited because its write current has been limited with respect to the set point, as mentioned above.
Attenuation of the reproducing output of a video signal in FIG. 1A is due to the loss of thickness in a magnetic recording medium (or tape) as its write current increases in succession. In other words, the magnetizing state varies with respect to the recording signal. When the video write current increases beyond saturation, since the magnetizing state of the magnetic recording medium is supersaturated, the reproducing output of the video signal is attenuated due to the loss of thickness in the magnetic recording medium.
It will be appreciated that the write current is always constrained by the set point, even when, as in an exclusive audio recording mode, a corresponding video signal is not received or, if received, is not to be recorded. Thus, the write current is constrained by the set point even when it is undesirable to do so as it will decrease the dynamic range of the signal of interest, the audio signal, being recorded.
Another problem associated with the conventional hi-fi VTR device is that no exclusive audio recording mode is possible for when a signal source applied for recording is supplied via an internally installed tuner.